Dispersants based on polybutene succinimides are ubiquitous in lubricating oils worldwide. An essential step in the manufacture of these dispersants is maleation of polybutene to polybutene succinic anhydride, which is further aminated with a polyamine to produce the succinimide. The maleation process is facilitated by polyisobutylene (PIB) with a high level of vinylidene at the polymer terminus, which can react smoothly with maleic anhydride in a thermal or “ene”, without the need for chlorine as a facilitator. Such polymers, termed highly reactive PIB or HR-PIB, are preferred in industry for their process, performance and environmental benefits.
Existing processes to manufacture HR-PIB via cationic polymerization of isobutylene (IB) utilize a BF3 catalyst associated with a range of alcohols and/or ethers often requiring pure IB feed, very low temperatures, and fluorine clean-up steps (Mach et al. Lubr. Sci. 1999 11(2) 175-185). Such processes can be costly in terms of both equipment and operations. More recently, catalysts based on AlCl3 or alkyl AlCl2 Lewis acids (LA) complexed with ether Lewis bases (LB) have shown the capability to produce HR-PIB in the presence of alkyl halide initiators (see review by Kostjuk, RSC Adv., 2015, 5, 13125-13144). The Lewis acid ionizes the initiator to start polymerization, and the Lewis base deprotonates the polymeric carbenium to the desired olefin. The binding strength of the LA.LB complex is dependent on the nature of LA and LB as well as on the environment. In a relatively polar medium such as dichloromethane (DCM), a Lewis acid.Lewis base complex of EtAlCl2 (EADC) and di-isopropyl ether is able to produce HR-PIB with high vinylidene levels. However, in the apolar, all hydrocarbon environment of a C4 polymerization reactor, the above complex is too strong and the Lewis acid is unable to ionize t-butyl chloride (t-BuCl), a common initiator used to start polymerization of IB.
Adjustment of the complexation energetics via introduction of electron withdrawing groups into the Lewis base as in bis-(2-chloroethyl) ether (CEE), results in efficient ionization of t-BuCl and initiation of IB polymerization even in apolar media, while still maintaining high vinylidene content (U.S. Pat. No. 9,156,924). The amount of reactive vinylidene olefin (exo olefin) produced is dependent on the rate of deprotonation of the polymeric carbenium ion relative to the rate of isomerization to more hindered ions that lead to less reactive olefins. Therefore, a high concentration of the CEE deprotonation agent is normally used to maximize the vinylidene content (e.g. 1.5-3 times the LA concentration).
A high concentration of CEE, however, can decrease the rate of IB polymerization because it inhibits dissociation of the LA.CEE complex which dissociation is required to ionize the t-BuCl initiator. Long residence times and large reactors would then be required to realize commercially reasonable monomer conversions. The high rate of deprotonation also lowers the molecular weight (MW) of the polymer product because it interrupts chain growth. Therefore, a means to simultaneously achieve high terminal vinylidene, high monomer conversion and high MW simultaneously using aluminum based catalysts at reasonable temperatures would be very valuable to the industry.
U.S. Pat. No. 7,411,104 teaches the addition of tertiary alcohols such as t-butanol in combination with a secondary alkyl ether, such as di-isopropyl ether, during the polymerization of IB using a BF3 catalyst to enhance the vinylidene content of the resulting HR-PIB polymer. This type of ether does not work with an aluminum based system, however, because the Lewis acid-Lewis base complex is too strong and polymerization is impeded (Macromolecules 2014, 47, 1959-1965). U.S. Pat. No. 7,411,104 further teaches that the ratio of secondary alkyl ether to tertiary alcohol used in that process must be in the range of 0.5-1.2 and if the mole ratio is “less or more than the above-mentioned range, the content of the terminal vinylidene decreases, and the advantageous effect of the present invention cannot be achieved”. In an aluminum-based system, however, such a large amount of tertiary alcohol would act as a poison, reducing conversion to low and commercially impractical levels.